1. Field of the Invention
The present invention relates to a support, a photomask blank and a light-sensitive photomask material. More particularly, it relates to a support suitable for spin coating, a light-sensitive photographic material which comprises a silver halide photographic emulsion layer formed on the support, a mask blank which comprises a mask layer formed on the support and a light-sensitive photomask material which comprises a silver halide photographic emulsion layer formed on the mask blank. A process of forming spin coated layers is also disclosed.
2. Descripion of the Prior Art
It is well known in the field of semiconductor device fabrication to coat a photoresist using an organic solvent as a coating solvent on a support by spin coating. For example, a support (e.g., a glass plate having a thin metal layer thereon, a silicon wafer, etc.) to be coated is placed on the turntable of a spinner and a photoresist coating solution is dropped thereon, and then the turntable is spun at a high speed (usually 2,000 to 6,000 r.p.m. ) to obtain a photoresist coating layer having a uniform thickness.
However, when a gelatin silver halide emulsion is coated on a support by spin coating, it has been found that the following problems arise.
The amount of binder (mainly gelatin) of a gelatin silver halide emulsion coating solution is usually much smaller than the solid content of a photoresist coating solution, therefore, the thickness of the silver halide emulsion layer after drying is much smaller than the thickness of the silver halide emulsion layer immediately after coating. Usually, it is decreased to about 1/50 its original value. Therefore, the thickness of the gelatin silver halide emulsion layer becomes extremely small (a fraction of a micron) if the spin coating is carried out at a high spinning speed as is in the spin coating of a photoresist.
Further, a gelatin silver halide emulsion gels at around room temperature, and if the spinning speed is high, the emulsion gels during spinning since the emulsion is chilled by the air stream generated by spinning.
Occasionally, the coated layer at the periphery is spattered since a high centrifugal force acts on the coated layer at the periphery. There is no problem if the coated layer at the periphery is spattered to uncover the surface of the support at the periphery completely and at a uniform width; however, the position and the size of the coated layer spattered are not uniform, and the emulsion remains on the uncovered surface of the support in a dappled fashion.
On the other hand, if the spinning speed is decreased (to 100 to 200 r.p.m.) to remove the above described defects, the following problems arise. First, when the spinning speed is decreased, the centrifugal force acting on the coated layer at the periphery of the support is small, and the coating solution collects on the periphery and side edges (particularly on corners when the support is square or rectangular) of the support, whereby the thickness of the coated layer at these portions becomes particularly large. This effect is particularly large as compared with photoresists, since the wettability of a gelatin silver halide emulsion coating solution to a support is poorer than that of a photoresist coating solution. The wettability of a gelatin silver halide emulsion to a support can be improved by adding a surface active agent thereinto, but is still insufficient. In addition, a large amount of surface active agent cannot be added, since the adhesion of the emulsion layer to the support becomes poor and other photographic properties are degraded. It may be considered that the thickness of the coated layer at the periphery becomes large as described above because the wettability of the emulsion to the support is poor and the viscosity of the emulsion becomes extremely high due to the temperature decrease during spinning.
It is not desirable that the thickness of the coated layer be large at the periphery of the support, since a non-uniform gap is formed between the photomask and the coated layer when a photomask, for example, is superimposed on the coated layer and exposed to light.